Screw compressors and expanders are composed of meshing screw or helical rotors. As in the case of gears, screw rotors have pitch circles which represent locations of equal tangential velocity for conjugate pairs of rotors. These spiral grooves in the rotors are the locations of the volumes of gas which are trapped and in the case of compressors, compressed due to the coaction of a conjugate pair of rotors and an enclosing casing. Accordingly, the volumes of the spiral grooves are a major design consideration, and their width, depth, length and number are important design variables. The shape of a cross section of the spiral grooves includes the variables of width and depth, as well as the shape requirements for the driving/driven coaction between the conjugate pair of rotors. Additionally, the conjugate pair of rotors must meet the sealing requirements as the line contact advances along the rotor profile in the driving/driven coaction and as the rotor tips and end faces coact with the enclosing casing. The line contact follows the perimeters of the rotor profiles and is therefore at a varying tangential speed and has significant radial components. Additionally, the shape and the cross section of the spiral grooves must meet requirements for ease of manufacture and cutting tool life. One problem associated with conventional screw rotor designs is that rotor profiles have generally been designed using a point generated and or circular profiles. These types of profiles are generally more difficult to machine, as well as exposing the rotors to more significant impact with respect to seal line length, drive band contact stress, service life, and sensitivity to temperature fluctuations.
There exists a need therefore for a screw rotor profile for reducing seal line length, reducing contact stress, increasing service life, and exhibiting more flexibility to temperature fluctuation.